This invention relates in general to concentration detectors and relates more particularly to a single mode diode laser that produces light at a wavelength .lambda. that is selected with sufficient accuracy and stability to be suitable for use in spectrometry.
Oxygen concentration sensors are useful in hospitals to monitor the concentration of oxygen in prenatal care and in other environments in which the oxygen concentration is critical. Oxygen can be poisonous if supplied in too high a concentration, so it is important that the oxygen concentration be neither too high nor too low. At the present time, an oxygen sensor is available that measures the oxygen concentration by means of an oxygen-reduction reaction. The oxygen oxidizes an electrode of an electrochemical cell, thereby releasing electrons that produce a current level proportional to the concentration of the oxygen. Unfortunately, this measurement can take on the order of a half minute and therefore is much slower than would be desired. Therefore, it would be advantageous to have an oxygen sensor that measures the oxygen concentration in a much shorter period.
Sample concentrations are also measured in the field of spectrometry. In spectrophotometry, light is passed through a sample to a detector and the spectral distribution of absorption is measured. The concentration of a particular substance in a sample under test is measured by measuring the height of a spectral peak located at a peak in the absorbance spectrum of that substance.
Soon after lasers were first developed it was appreciated that lasers are particularly suitable for use in spectroscopy because they produce extremely narrow light beams having an extremely high energy density. This enables a high concentration of light to be passed through a very small sample, thereby enabling spectroscopic measurements to be performed on microliter sized samples. A laser beam can also be very monochromatic, as is needed for high resolution spectroscopic measurements.
Unfortunately, most lasers cannot be tuned over a significant spectral range so that such lasers cannot be used for detecting a wide range of substances. Although dye lasers, certain solid state lasers (such as Al.sub.2 O.sub.3 :Ti.sup.3+ solid state lasers) and semiconductor lasers can be tuned over a useful range of wavelengths, most other lasers are used as light sources only at a discrete wavelength. This discrete wavelength is determined by the energy band structure of the lasing medium and does not typically coincide with the wavelength of interest in detecting most substances. Therefore, most spectroscopic and spectrometric systems utilize tunable lasers.
Of the three above-mentioned tunable lasers, semiconductor lasers (also referred to as diode lasers) are the most attractive for commercial spectrometers because of their simplicity of operation (i.e., they are pn junctions), their small size, their wide range of tunability and their relatively low cost. In general, diode lasers can be tuned by variation of drive current through the diode and by variation of the diode temperature. Another factor making diode lasers attractive for spectrometry is that, over the past two decades, a great amount of research has been directed to producing inexpensive, dependable, single mode diode lasers.
Most of this research has been motivated by telecommunication applications in which single mode lasers are needed to produce a carrier for transmission through optical fibers. In such applications, it is important that this monochromatic light beam carry light of a wavelength that is not strongly attenuated by the optical fibers. This has resulted in the selection of a few standard wavelengths for use in telecommunications and this in turn has directed much of the research to the improvement of devices that operate at these standard wavelengths.
AlGaAs diode lasers at around 820 nm and InGaAsP diode lasers at 1.30 .mu.m and 1.55 .mu.m received a substantial amount of research efforts because they take advantage of the optical fiber absorption minima at these wavelengths. These diodes exhibit excellent operational characteristics, including high monochromaticity (0.01-0.1 .ANG. FWHM), low threshold current (typically on the order of tens of milliamps), excellent output power, excellent frequency stability, high modulation rate (up to 3 GHz) of the drive current and long life (greater than 100,000 hours cw) while operating at up to 50.degree. C. Unfortunately, since this research was directed to telecommunications applications, much of this work is not necessarily of use in spectrometry.
Lasers are optical oscillators that can potentially produce light at any of the resonant wavelengths of the optical cavity modes. Since the gain spectrum of a laser is typically much wider than the wavelength spacing between cavity modes, more than one mode can experience a net gain of at least unity. Each of such net gain modes will then grow until it saturates. Lasers can therefore operate in three different modes of operation: modelocked, multimode and single mode.
In modelocked operation (also referred to as phaselocked operation), only two modes are allowed to lase and are phaselocked so that the output intensity of these two interfering signals exhibits a sinusoidal modulation at the difference frequency of these two lasing modes. In multimode operation, at least two cavity modes are allowed to lase, but the relative phase between each mode varies incoherently so that the output signal does not exhibit modulation at the difference frequencies. In single mode operation, the laser is configured to allow only a single mode to lase. This is achieved by damping all but one of the modes of the laser cavity that would otherwise exhibit a net gain greater than one.
There are two widely used methods of damping unwanted modes: coupled cavity and distributed feedback (DFB). In a DFB type single mode laser, the thickness of one of the layers of the laser is varied spatially at a period equal to the wavelength of the single mode that is to be selected for lasing. The resulting periodic perturbation of the refractive index provides mode selective feedback by means of backward Bragg scattering. Unfortunately, the period of variation of layer thickness is fixed at the time of manufacture so that this type of laser cannot be tuned to an absorption peak in the substance to be detected. Also, the manufacturing process does not produce a period of thickness variation with sufficient repeatability to make these lasers suitable for use in spectrometry. A very limited amount of tuning can be produced by variation of the drive current and/or the temperature of the diode laser, but a very elaborate temperature control system would be required to select a wavelength within the few tenths of an Angstrom accuracy needed for gas spectrometry. This would require control of the temperature to an accuracy of less than 0.05.degree. C.
In a coupled cavity type single mode laser, an external cavity provides the wavelength dependent feedback needed for single mode operation. This external cavity can be either pumped or unpumped. When the external cavity is pumped, this introduces an additional degree of freedom that can be used to control the behavior of the laser. This external cavity introduces damping that varies periodically with wavelength. The mode selected for single mode operation is the mode having the greatest net gain and is typically the mode having the lowest cavity loss closest to the peak of the laser gain profile.
These two types of single mode diode lasers exhibit superior performance for many applications, but the cost and complexity of their operation are justified only in applications, such as telecommunications, where the benefits justify the increased cost. Of greater importance to spectrometric applications, these single mode lasers generally have limited tunability. Because of the narrowness of the absorption peaks in spectrometry, suitable optical sources should not only provide tunability, but should also provide the accuracy and stability of wavelength selection needed to select a laser wavelength that accurately matches the wavelength needed for sample measurement. The telecommunication application toward which most prior work was directed does not require such accurate, stable wavelength selection.